Gently clean the mushrooms with a damp cloth and carefully remove the stems. Chop the stems finely.
In a skillet over medium heat, add olive oil and butter. Sauté the chopped mushroom stems for about 3-5 minutes until they are softened.
Add the chopped onion and bell pepper to the skillet. Cook for another 2-3 minutes until they become translucent. Stir in the minced garlic and cook for 1 minute. Remove from heat and add the minced clams, breadcrumbs, parmesan cheese, basil, oregano, salt, and pepper. Mix until well combined.
Fill each mushroom cap generously with the stuffing mixture.
Combine mozzarella, romano, and parmesan cheeses in a bowl. Sprinkle the cheese mixture over the stuffed mushrooms.
Place the stuffed mushrooms in a small baking dish. Bake in the preheated oven for about 15 minutes, or until the mushrooms are tender. Broil for the last 2-3 minutes of cooking to achieve a golden brown finish. Watch carefully to prevent burning.
Garnish with freshly chopped parsley and serve with lemon wedges on the side, if desired.
Notes
* If you use smaller mushrooms, you will have leftover filling and cheese topping to make more than 12. With smaller mushrooms, you can make about 24-30 stuffed mushrooms.